The present invention relates to a push button switch which may be used for an audio device mounted on a motor vehicle, camera, facsimile receiver, VTR device, and others.
FIG. 5 is a sectional view showing a conventional push button switch disclosed in the publication of Japanese Utility Model Laid Open 58-135830, and FIG. 6 is a plan view showing an internal structure of the switch. The switch comprises a case 1, movable contact plate 7 provided in the case 1, fixed contact plate 7a embedded in the bottom of the case 1, and push button 3.
The fixed contact plate 7a comprises a U-shaped first plate 7b and a straight second plate 7c. The first plate 7b has a pair of exposed first fixed contacts 4a and 4b, and the second plate 7c has an exposed second fixed contact 5a. The first plate 7b has a terminal 4, and the second plate 7c has a terminal 5.
As shown in FIG. 7, the movable contact plate 7 comprises an upwardly arcuated movable contact 10, a pair of side plates 11a and 12a, having downwardly arcuated permanent contacts 11 and 12, respectively. Each of the contacts 11 and 12 permanently contacts with one of the first fixed contacts 4a and 4b. A pair of slits 8 and 9 are formed between the movable contact 10 and the side plates 11a and 12a, and connecting portions 13 and 14 are provided at both ends.
The push button 3 has a spherical projection 3b at the underside thereof, contacting with the movable contact 10. The push button 3 is upwardly urged by the movable contact 10 and projected from a cover 2 through a window of the cover, so that a flange 3a is pressed against the underside of the cover 2.
When the push button 3 is depressed by a finger of a user, the movable contact 10 of the movable contact plate 7 is pushed down to the horizontal position. However, the shape of the other portions except for the movable contact 10 is not changed. When the movable contact 10 passes the horizontal neutral position, the movable contact 10 is inverted into the reverse arcuated position with a click, so that the contact 10 contacts with the second fixed contact 5a. Thus, the switch is closed. When the finger of the user is detached from the push button 3, the movable contact 10 returns to the initial position so that the switch is opened.
In such a push button switch, it is necessary that the movable contact 10 is instantaneously inverted with a click. To this end, other portions than the movable contact must be securely held, and must have sufficient stiffness. However, each of the contacts 11 and 12 has the same thickness as the movable contact 10 and has stiffness approximately equal to that of the movable contact 10. Therefore, when the push button 3 is depressed, the contacts 11, 12 are slightly extended to absorb a part of the depressing force. The inversion action of the movable contact 10 is accordingly dulled. By repeatedly using the push button switch, the fatigue of the contacts 11, 12 increases, thereby aggravating the function of the switch.
There is a further problem that the contacting pressure of the contacts 11 and 12 against the first fixed contacts 4a and 4b is low because of the plane contact there-between, which decreases the reliability of the electric conductivity.
Japanese Utility Model Laid Open 59-23119 discloses a movable contact plate for resolving the above described problems. FIG. 8 is a perspective view of the movable contact plate. In the contact plate, each of the bending lines 15, 16, 17 and 18 of the contacts 11 and 12 is inclined angle e with respect to the outside edge. Consequently, the stiffness of the contacts 11, 12 somewhat increases compared with the contacts of FIG. 7, and the contact area of each contact, becomes smaller because the surface of the contact is inclined with respect to the first fixed contact 4a (4b). However, the effect is not satisfactory for the push button switch.
In addition, the width W2 of the fixed contact plate 7a must be larger than the width W1 of the movable contact plate 7.